1. Field of the Invention
The invention relates to an electrophotographic photoconductor to be used for image formation by electrophotography and an image forming apparatus provided with the electrophotographic photoconductor.
2. Description of the Related Art
An electrophotographic image forming apparatus to be used for a copying machine, a printer, a facsimile apparatus (hereinafter, referred to as electrophotographic apparatus in some cases) or the like forms an image through the following electrophotographic process. At first, a photosensitive layer of an electrophotographic photoconductor (hereinafter, simply referred to as photoconductor in some cases) installed in the apparatus is evenly charged at a prescribed potential by an electric charger and exposed to light such as laser beam radiated by exposure means corresponding to image information to form an electrostatic latent image. Next, a developer is supplied to the formed electrostatic latent image from development means for depositing colored fine particles called as a toner, which is a component of the developer, on the surface of the photoconductor to develop the electrostatic latent image and visualize a toner image. Further, the formed toner image is transferred onto a transfer material such as recording paper from the surface of the photoconductor by transfer means and fixed by fixing means.
At the time of transfer operation by the transfer means, the toner on the photoconductor surface is not necessarily entirely transferred and shifted to the recording paper, but partially remains on the photoconductor surface or the paper powder of the recording paper brought into contact with the photoconductor at the time of transfer may remain while being stuck to the photoconductor surface.
The remaining toner and the foreign substances such as the adhering paper powder on the photoconductor surface cause a bad effect on the quality of an image to be formed and therefore they are removed by a cleaning apparatus.
In recent years, cleaner-less techniques have been advanced and the remaining toner is recovered by cleaning function added to development means with no use of exclusive cleaning means: that is, the remaining toner is removed by a cleaning system simultaneously with development. Next, after the cleaning of the photoconductor surface in that manner, static electricity of the photosensitive layer surface is removed by a static eliminator or the like, thereby eliminating the electrostatic latent image.
The electrophotographic photoconductor to be used in such electrophotographic process is composed by layering a photosensitive layer containing photoconductive material on a conductive substrate made of a conductive material. As the electrophotographic photoconductor has been conventionally used an electrophotographic photoconductor using an inorganic photoconductive material (hereinafter, referred to as inorganic photoconductor).
Typical examples of the inorganic photoconductor are selenium type photoconductors having a photoconductive layer containing such as amorphous selenium (a-Se) or an amorphous selenium-arsenic (a-AsSe); zinc oxide type or cadmium sulfide type photoconductors having a photoconductive layer containing zinc oxide (ZnO) or cadmium sulfide (CdS) dispersed together with a sensitizer such as a coloring material in a resin; and amorphous silicon type photoconductors, having a photoconductive layer containing amorphous silicon (a-Si) (hereinafter, referred to as a-Si photoconductor).
However, the inorganic photoconductor has the following disadvantageous points. That is, the selenium type photoconductors and the cadmium sulfide type photoconductors are problematic in the heat resistance and storage stability. Further, selenium and cadmium are toxic to living things including human being and their use is a problem in terms of the environmental pollution and therefore, it is required to collect the photoconductors using them and to properly dispose them after use. Moreover, the zinc oxide type photoconductors are disadvantageously less sensitive and inferior in durability and therefore, they are scarcely used today.
On the other hand, the a-Si photoconductors drawing attention as an environment-friendly inorganic photoconductor are advantageous having high sensitivity and good durability, however since they are disadvantageously produced by a plasma chemical vapor deposition method, it is difficult to evenly form a photosensitive-layer and image defects are easily caused. Further, the a-Si photoconductors are inferior in the productivity and thus the production cost is disadvantageously high.
As described, since inorganic photoconductors have many disadvantageous points, it has been required to develop new photoconductive materials to be used for the electrophotographic photoconductor and accordingly an organic type photoconductive material, that is an organic photoconductor (Organic Photoconductor: abbreviated OPC), has been often used in place of the conventionally used inorganic type photoconductive material.
The electrophotographic photoconductor using an organic type photoconductive material (hereinafter, referred to as organic photoconductor) is rather much advantageous as compared with the inorganic photoconductor in terms of toxicity, production cost, and option of the material planning although having slight problems in the sensitivity, durability, and environmental stability. Further, the organic photoconductor has an advantageous point that its photoconductive layer can be formed by an easy and economical method, for example, by an immersion coating method.
Having many advantageous points as described above, the organic photoconductor tends to be used dominantly for the electrophotographic photoconductor. Further based on the recent investigations and developments, the sensitivity and durability of the organic photoconductor have been improved and today the organic photoconductor has been used for the electrophotographic photoconductor, except special cases.
Particularly, the capability of the organic photoconductor has been remarkably improved by development of a function-separation type photoconductor in which the charge generating function and the charge transporting function are allotted to respectively different substances. That is, the function-separation type photoconductor has an advantageous point in addition to the above-mentioned advantages of the organic photoconductor that the option of selecting materials composing the photoconductive layer is wide and that the production of a photoconductor having desired characteristics is relatively easy.
This function-separation type photoconductor is classified into a layered type and a monolayer type and in a layered type function-separation photoconductor, a layered type photoconductive layer composed of a charge generating layer containing a charge generating substance to which the charge generating function is allotted and a charge transporting layer containing a charge transporting substance to which the charge transporting function is allotted.
The above-mentioned charge generating layer and charge transporting layer are, in general, formed by dispersing the charge generating substance and the charge transporting substance respectively in binder resins, which are binders.
On the other hand, the monolayer type function-separation photoconductor has a monolayer type photoconductive layer formed by dispersing the charge generating substance and the charge transporting substance together in a binder resin.
As the charge generating substance to be used for the function-separation type photoconductor have been investigated many kinds of substances such as phthalocyanine pigments, squarylium coloring materials, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine coloring materials, squaric acid dyes, and pyrylium type coloring materials and various kinds of materials with high light fastness and high charge generating capability have been proposed.
As the charge transporting substance have been developed pyrazoline compounds, hydrazone compounds, triphenylamine compounds, stilbene compounds and moreover, in recent years, pyrene derivatives, naphthalene derivatives, and terphenyl derivatives having condensed polycyclic hydrocarbons as a center mother skeletone have been developed.
A charge transporting substance is required    (1) to be stable to light and heat;    (2) to be stable to active substances such as ozone, nitrogen oxide (NOx), and nitric acid generated by corona discharge at the time of charging a photoconductor;    (3) to have high charge transporting capability;    (4) to have high compatibility with an organic solvent and a binder resin; and    (5) to be produced easily at a low cost.
However, the above-mentioned conventionally known charge transporting substances satisfy some of these requirements but cannot satisfy them at high level.
Recently, the photoconductor has been required to have high sensitivity as a photoconductor characteristic and the charge transporting substance is required to have particularly high charge transporting capability corresponding to the demands for miniaturization and high speed to electrophotographic apparatus such as digital copying machines and printers. Further in the high speed electrophotographic process, since the time from exposure to development is short, it is required for the photoconductor to be excellent in the photo-response.
If the photo-response of the photoconductor is low, that is, if the decaying speed of the surface potential after the exposure is slow, the remaining potential rises and the photoconductor is used repeatedly in the state that the surface potential is not sufficiently decayed and the surface charge to be removed is not sufficiently eliminated by the exposure to result in undesirable consequence such as early deterioration of the quality of images.
On the other hand, in the function-separation type photoconductor, the charge generated in the charge generating substance by light absorption is transported to the photosensitive layer surface by the charge transporting substance and the surface charge in the portion of the photoconductor radiated with light is removed, so that the photo-response depends on the charge transporting capacity of the charge transporting substance. Accordingly, also in terms of actualization of a photoconductor having sufficient photo-response, the charge transporting substance is required to have high charge transporting capability.
As the charge transporting substance satisfying the above-mentioned requests have been proposed enamine compounds having higher charge transporting capability than that of the above-mentioned conventionally known charge transporting substances (e.g. reference to Japanese Patent Application Laid-Open (JP) No. Hei 2-51162, JP No. Hei 6-43674 and JP No. Hei 10-69107). Further, to improve a hole transporting capability of a photoconductor, addition of a polysilane and an enamine compound having a specified structure to a photosensitive layer is also proposed (e.g. reference to JP No. Hei 7-134430).
In actual use of an electrophotographic apparatus, since the above-mentioned operations of charging, exposure, development, transfer, cleaning, and static elimination are repeated for the photoconductor under various conditions, the photoconductor is required to have environmental stability, electric stability, and durability to external mechanical force in addition to the high sensitivity and excellent photo-response.
Practically, it is required for the photoconductor to have a surface layer hard to be abraded by sliding and friction with a cleaning member or the like. Accordingly, it is made possible to provide a photoconductor excellent in high durability to printing by specifying physical properties of the photoconductor surface satisfying the above-mentioned aims.
Hardness is one of indexes for evaluation of the physical properties of a wide range of materials including the electrophotographic photoconductor surface, particularly for evaluation of mechanical properties. Hardness is defined as the stress of a material when a presser is pushed into the material. It is tried to quantify a mechanical property of a film composing the electrophotographic photoconductor surface by using the hardness as a physical parameter informing the physical property of a material. For example, a scratching strength test, a pencil hardness test, and a Vickers hardness test have been known well as a testing method for measuring the hardness. However, in any hardness test, there are problems in measurements of mechanical properties of a material showing complicated behaviors such as plasticity, elasticity (including delay component) as a film containing an organic matter, or the like.
For example, Vickers hardness test evaluates the hardness by measuring the length of the pressed trace in a film, however it reflects only the plasticity of the film and it cannot precisely evaluate a mechanical property of organic matter which may be deformed at a high elastic deformation ratio. Accordingly, mechanical properties of a film made of organic matter have to be evaluated in consideration of various characteristics.
In the electrophotographic photoconductor having an organic photosensitive layer as the surface layer, for example, plastic power (plastic deformation ratio, ηplast, %) and elastic power (elastic deformation ratio, ηHU, %) are described as the physical properties to be used for judgment of long term abrasion resistance, durability, and operational stability (e.g. reference to JP 2000-10320 and JP 2002-6526).
The plastic power is a percentage of the ratio of the plastic deformation workload to the total of the plastic deformation workload (energy required for the plastic deformation) and the elastic workload (energy required for the elastic deformation).
Further, the elastic power is a percentage of the ratio of the elastic deformation workload to the total of the plastic deformation workload and the elastic workload.
Accordingly, the total of the plastic power and the elastic power becomes 100 (%).
JP 2000-10320 practically describes that the plastic power (plastic deformation ratio) is set to be in a range from 30 to 70% and that the universal hardness (Hu) measured by a universal hardness test standardized in DIN50359-1 is set in a range from 230 to 700 N/mm2. Further, the Document No. 5 describes that such setting in the numerical range prevents mechanical deterioration of the photoconductor surface layer.
However, the numerical range of the plastic power from 30 to 70% is a range covering almost all of organic photosensitive layers containing binder resins used commonly today. Accordingly, even if the plastic power is in the above-mentioned range, it is not necessarily always possible to obtain an organic photosensitive layer excellent in long term abrasion resistance, durability, and operation stability.
Further, JP 2002-6526 described an electrophotographic photoconductor comprising an organic photosensitive layer and a protection layer containing a curable resin as a binder resin on a conductive support and having an elastic powerηHU (=[plastic deformation workload/(plastic deformation workload+elastic workload)]×100) of the protection layer in a range from 32 to 60%.
However, the numeral value of 32 to 60% for the elastic power means the same as that the plastic power is in a range from 40 to 68% and similarly to JP 2000-10320, it covers almost all of electrophotographic photoconductors having an organic photosensitive layer as the surface layer which have been used today.
Further, the curable resin to be used as the binder resin is also common in technical fields of electrophotographic photoconductors.
Accordingly, JP 2002-6526 does not practically describe the solution means of obtaining an organic photosensitive layer excellent in long term abrasion resistance, durability, and operation stability. Further, the electrophotographic photoconductor of JP 2002-6526 has a problem that formation of the protection layer containing the curable resin leads to the cost up.
Conventionally, it has been tried to increase the ratio of a binder resin to be used for the surface layer or use a resin with a high molecular weight in order to heighten the durability of the electrophotographic photoconductor as the means for increasing the durability to printing. However increase of the resin ratio results in decrease of the relative amount of a charge transporting material in the surface layer and decrease of the sensitivity of the photoconductor and thus it is unsuitable for recent tendency of speed acceleration.
As means for overcoming such defective points, it is proposed to further separate the function of a charge transporting layer and add a type of resin excellent in durability more in the most outer surface layer and compensate sensitivity for a layer underneath (e.g. reference to JP 2000-214602). However, there is no disclosure of surface properties practically controlling the durability. Further, use of a binder resin with a high molecular weight causes increase of the viscosity of a coating solution in an immersion coating method to result in a problem of decreasing productivity.
Further, a polyarylate type resin described in JP 2004-219922 is disclosed to have a type of resin excellent in exhibition of high printing durability, however in terms of the solubility, it is indispensable to use a halogenated benzene such as monochlorobenzene and a specified halogen type organic solvent and in terms of the effect on the health of human being and global environmental preservation, it cannot be denied that the production is limited considerably.